1. Field of the Invention
The present invention relates to a hydraulic pressure system, and more particularly to a hydraulic pressure system including an accumulator having improved pressure characteristics and an articulated mechanism having a link or an arm serving as a high-pressure container made up of a composite material.
2. Description of the Prior Art
Accumulators are widely employed in hydraulic pressure systems for the purpose of saving energy or lowering vibration. As shown in FIG. 3 of the accompanying drawings, an accumulator 3 is connected to an oil supply passage interconnecting an oil pressure pump 1 and an actuator 2. When the amount of oil consumed by the actuator 2 exceeds the amount of oil supplied by the pump 1, the accumulator 3 supplies assistive oil pressure to the actuator 21, thus allowing the capacity of the pump 1 to be reduced and saving oil pressure energy. The accumulator 3, which is shown as being of the bladder type, has a pressure-resistant container 6 and a rubber bag 4 disposed in the container 6 and filled up with a high-pressure gas. When the oil pressure for operating the actuator 2 is higher than the pressure of the filled gas in the bag 4, working oil supplied from the pump 1 is stored in a space 5 defined between the container 6 and the bag 4 while compressing the gas in the bag 4 to reduce the volume of the gas. When the capacity of the pump 1 to supply oil pressure becomes insufficient, the stored oil pressure energy is discharged from the accumulator 3 to ensure smooth operation of the actuator 2. One example of such accumulator 3 is disclosed in Japanese Laid-Open Patent Publication No. 57-37101, for example.
The relationship between the volume and pressure of the stored working oil is governed by the Boyle's law and the Charles' law such that the pressure is in inverse proportion to the volume of the gas chamber defined by the bag 4. FIG. 4 of the accompanying drawings shows the relationship between the volume and pressure of stored working oil. The solid-line curve in FIG. 4 shows the pressure vs. volume characteristics of an accumulator having a relatively small capacity, indicating that the pressure P rapidly increases as the volume V of the stored oil increases. In determining the maximum capability of a hydraulic system, it is ideal that the rate of flow of supplied oil and the pressure thereof be given minimum settings. If the oil pressure supplied from an accumulator used as an assistive oil pressure source in the hydraulic system varies, then various problems occur. First, when the oil pressure from the accumulator is higher, the pump performs more work than necessary, resulting in uneconomical operation. Secondly, when the oil pressure from the accumulator is higher, more oil flows through the opening of a control valve to the actuator, so that control may be not reliable. Thirdly, the hydraulic system has to be of more mechanical strength than required to provide against higher accumulator pressures.
To eliminate the above shortcomings, it has been customary to employ an accumulator of a relatively large capacity to keep the pressure constant without variations even when the volume of the stored working fluid or oil is increased, as indicated by the broken line in FIG. 4. More specifically, the oil pressure Po from the accumulator is set to So' rather than So with respect to the volume Vo of the stored oil. The accumulator has a pressure-resistant container having a large wall thickness and made of an iron-base material having a large mechanical strength. The larger the accumulator, therefore, the heavier and the bulkier the accumulator when it is incorporated in a hydraulic pressure system. Another drawback with accumulators of the bladder type is that a gas cannot be kept in a rubber bag for a long period of time and tends to pass through the membrane or bag wall into the working oil contained in the oil chamber of the accumulator. The larger the accumulator, the greater the bag, i.e., the greater the surface area of the bag, allowing more gas to pass through the bag.
A variety of articulated mechanisms such as industrial robots for use in factories have been proposed in recent years. One such articulated mechanism is disclosed in Japanese Laid-Open Patent Publication No. 52-69152, for example.
The articulated mechanism employ many lightweight materials to reduce the inertial efficiency of articulated arms for allowing higher-speed operation. The most popular lightweight material for use in the articulated mechanisms is a fiber-reinforced synthetic resin, i.e., a high polymer composite material such as CFRP or the like. Where the arms of links of an articulated mechanism are made of such a composite material, the articulations of the mechanism which are interconnected by the links are made of a metal material such as aluminum, for example, which is highly durable and allows an actuator such as an electric motor or a hydraulic motor to be mounted easily therein. Various efforts have been made to connect a metal material and a composite material suitably to each other. One problem which is frequently encountered with the joining of such two different materials is that the coefficients of thermal expansion of the materials are largely different from each other. More specifically, the composite material needs to be treated at a high temperature when it is shaped for connection to the metal material. When the composite material is cooled down to a stable point, since its coefficient of thermal expansion is different from that of the metal material, excessive residual stresses are developed in the composite material, with the results that the manufactured components have low mechanical strength and durability.
Where the articulated links are actuated under hydraulic or pneumatic pressure, the internal space of one of such links may be employed as an auxiliary gas chamber for an accumulator, which chamber stores the pressure of a high-pressure gas. In such a case, the high gas pressure in the link is applied to joints at the opposite ends of the link, thus tending to deform the composite material of the link. Inasmuch as the composite material has a relatively low modulus of elasticity, it is elastically deformed to a larger extent than the metal material, and may be destroyed at the joints of the link.
High-pressure containers include a spherical container such as a large-size gas tank and a cylindrical container such as a gas cylinder for home use or a cylinder of compressed air for use as an aqua lung. The latter cylindrical container is disclosed in Japanese Laid-Open Patent Publication No. 57-101195, for example.
The spherical container is large in size though the wall thereof is subject to small stresses and may be made of a lightweight and thin material. For this reason, many high-pressure containers are cylindrical in shape. Where the material of a cylindrical container is isotropic, stresses produced in the container wall under the pressure of a high-pressure gas filled in the container are larger in the circumferential direction than in the axial direction. Specifically, the circumferential stresses are about twice the axial stresses. The reason for this is given below. When an internal pressure P is applied to a cylindrical container having an inside diameter of d and a wall thickness of t, a fluid force Fl acting in the axial direction of the container is expressed by: EQU Fl=(.pi.d.sup.2).times.P/4 [kg]
By dividing this by the cross-sectional area S1=.pi.dt of the cylinder, a stress pl (in the axial direction) is given by: EQU .rho.l=dP/4t [kg/cm.sup.2 ]
Assuming that the container has a length L, a fluid force F2 tending to tear the cylindrical container apart in the circumferential direction is expressed as follows: EQU F2=dLP [kg]
and the cross-sectional area, in the direction of the thickness, of the wall of the container for resisting the force F2 becomes: EQU S2=2.times.Lt [cm.sup.2 ]
A circumferential stress p2 is thus given by: EQU p2=F2/S2=dP/2t=2.rho.1
Therefore, the circumferential stress is twice as large as the axial stress. This fact derives from the fact that the material of the container is isotropic. The high-pressure containers, particularly gas containers for home use, should preferably be lightweight. Attempts are being made to construct such containers from a fiber-reinforced synthetic resin or a composite material made primarily of a high polymer material. A better approach to the use of such a composite material in a high-pressure container is being sought.
It is known that the fibers of a composite material of a high-pressure container should be wound at an angle of 54.75 degrees with respect to the axis of the container. The composite material fibers are wound at such an angle in uniform directions by the filament winding process.
The reason why the fibers should be wound at 54.75 degrees will be described briefly with reference to FIGS. 7A, 7B, and 7C which show a relatively long cylindrical container.
Considering the balancing of forces in the circumferential direction, a cross-sectional area perpendicular to the direction of all fibers passing through a plane ab is given by: EQU S=(2.pi.r.multidot.cos.theta.)t [cm.sup.2 ]
Assuming that the stress imposed on the fibers is indicated by .rho., a force F applied in the direction of the fibers is expressed as follows: EQU F=.rho.t(2.pi.r.multidot.cos.theta. [kg](1)
Therefore, a force FC applied in the circumferential direction becomes: EQU FC=2pt(2.pi.r.multidot. cos .theta.) sin .theta. [kg](2)
As shown in FIG. 7C, a force FCP produced by an internal pressure P in the cylinder is: EQU FCP=2rP.multidot.2.pi.r.multidot.cot.theta.[kg] (3)
Since FC=FCP because these forces are balanced, the equations (2) and (3) are equal to each other, and the following equation is derived: EQU P=.rho.t.multidot. sin .sup.2 .theta./r [kg/cm.sup.2 ](4)
With respect to the balancing of forces in the axial direction, a force FA acting on the plane ab or a plane cd is given, as shown in FIG. 7b, as follows: EQU FA=.rho.t(2.pi.rcos.theta.)cos.theta. [kg](5)
A force FAP generated in the axial direction by the internal pressure P is expressed by: EQU FAP=.pi.r.sup.2 P [kg](6)
The equations (5) and (6) are equal to each other since they are balanced, and the following equation results: EQU P=2.rho.t.multidot. cos .sup.2 .theta. [kg](7)
Since P in the equation (4) and P in the equation (7) are the same, these equations are equal to each other, thus obtaining: EQU tan.sup.2 .theta.=2
Therefore, .theta.=54.75 degrees.
If a high-pressure container is to be of a smooth cylindrical shape without any projections, presenting no obstacle to the filament winding process, then a highly durable container can be manufactured since a long continuous filament is employed. If a high-pressure container having bulging axially opposite ends is to be manufactured, however, such a container configuration prevents the filament winding process from being relied upon in producing the container.